The Section is continuing its work on the molecular interaction between antigens and monoclonal antibodies. Elucidation of the nature of this interaction is of importance not only in immunology but for a general understanding of the interaction between receptors and haptens. Our approach is three-fold: A. The interaction of ligands (natural or synthetic) with monoclonal antibodies is evaluated and (possibly) correlated with epitopes on both antigen and protein. B. Rearranged immunoglobulin genes are cloned with the object of site specific mutagenesis so that specifically altered antibodies may be obtained. These can then be evaluated for their altered binding properties. C. The preparation of cold, and radioactively labeled affinity labels, and their reaction with monoclonal antibodies.